Needle-stick accidents are particularly prevalent and carry a high risk of infection in the field of phlebotomy (blood sampling). There is also a risk of needle-stick injuries during the use of syringes for injection of fluids. Syringes and blood samplers with provision for automatic needle retraction are disclosed in the applicant's earlier specification WO 92/018187. These devices, and other design variants subsequently developed, comprise a tubular hollow body of suitable plastics material in one end of which plural deflectable latches are integrally formed for the purpose of mounting a needle hub against bias of a spring.
In use, a plunger having a hollow interior and a closed rear end is inserted into the hollow body and pushed inwards until its front end forces apart the deflectable latches to release the spring, which drives the hub and an attached needle assembly rearwards into the interior of the plunger. This is now a well-known manner of automatically retracting and encapsulating a needle. All that is required is the insertion and full depression of the plunger. However, in some circumstances, the force required to be applied to the plunger in order to deflect the latches outwards and bring about needle retraction is considered unacceptably high. For example, single-handed operation of the device is preferable and not all relevant medical staff have sufficiently strong hands to manipulate with ease. It is, in any event, desirable to reduce the force required to trigger needle retraction in such devices in order to achieve smoother operation.
The preferred material for the hollow body of such devices is polypropylene owing to its good strength and suitability for moulding, particularly resistance to shrinkage during moulding. For this reason, and other reasons related to detailed standards set for production and operation criteria for such medical devices, it is not possible to change the material. It is also important that the latches do not lose required strength/stiffness for the initial retention of the needle hub against spring bias, including during application of needle point load during use of the device.
The deflectable latches are formed integrally with the hollow body, that is to say they are moulded in one piece. The latches extend longitudinally inside the hollow body from body walling at or adjacent a first end of the body. In the production process for certain known devices of the applicant a respective opening is formed radially inwards of each of the latches.